n,m,*a=map(int,open(0).read().split())
p=[]
for i,j in zip(a,a[1:]):
    if j>i+1:
        p+=(j-i-1),
ans=n-m
for t in p:
    y=t
    c=1
    while y%2 and y>1:
        y=(y-1)//2
        ans-=c
        c*=2
print(ans)